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Posted: Nov 08, 2010

'Nano-urchins' as anode material for greener batteries

(Nanowerk News) Montroseite, a vanadium oxide mineral first discovered 60 years ago, could be used as an anode for greener batteries, say Chinese researchers.

Yi Xie and co-workers from the University of Science and Technology of China Hefei, Anhui, China, synthesised montroseite, which was named after Montrose County in Colorado where it was discovered. The mineral they made shows improved electrochemical properties compared to nanorod electrodes because of its sea urchin-like structure, making it a better anode for aqueous lithium ion batteries.

Lithium ion batteries work by generating electricity through the movement of lithium ions between a negative electrode (anode) and a positive electrode (cathode). Aqueous lithium ion batteries use a water-based electrolyte (a chemical compound that conducts electricity when molten or dissolved in solution) and are a safer and cheaper alternative to electrolyte solutions in standard lithium ion batteries, which can be toxic and flammable. Xie says that aqueous electrolytes have high ion conductivities, which equates to high power densities. These conductivities are about two orders of magnitude higher than those of organic electrolytes, making aqueous lithium ion batteries promising 'green' batteries. However, aqueous electrolytes are not as stable as organic electrolytes so the choice of electrode is crucial to the battery's performance.

Xie's team prepared the montroseite using a simple hydrothermal reaction, resulting in montroseite with a hollow urchin-like structure. Each 'urchin' is made of one-dimensional nanorods pointing out from the centre of a sphere to resemble the sea creatures. The researchers also looked at the mineral's oxidation and dehydrogenation into the corresponding dioxide, paramontroseite, which also exists in nature (from the natural weathering of the ore).

The team found improved electrochemical performance for both montroseite and paramontroseite due to high lithium uptake and release suggesting that they could be used in anode materials. Xie credits these improved properties to the combined effects of the microscopic tunnelled crystal structure and the macroscopic hollow morphological features. These provide easy electrolyte infiltration, short diffusion lengths for lithium ions and electron transport, and sufficient void space to buffer the volume change during lithium uptake and release.

Xiaohong Zhang at the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, China, says that if the cycling performance can be further improved, this 'green' battery has the potential to boost progress in the development of energy storage and conversion systems.